Solar collector

ABSTRACT

A solar collector, includes a glass sheet; a metal frame or another glass sheet and a metal frame; a seal between the glass sheet or sheets and the metal frame; the metal frame including a wall offset with respect to the seal and/or a wall connected to the seal by a low thermal conductivity material. The invention provides a solar collector which is compact and simple and improves solar radiation transmission.

The invention relates to a solar collector.

Solar collectors absorb the heat from solar radiation by virtue of anabsorber. A heat-transfer fluid circulates in heat collector pipes fixedto the absorber. The pipes enable the heat to be transported to the userand make it possible to keep the absorber at a reasonable temperature.Such a collector may be used for example to heat water for domesticapplications, to supply thermal energy to a refrigeration unit forproducing conditioned air, to desalinate sea water or to purify waterfor the purpose of supplying drinking water, or else to dry materials inan industrial plant.

Document GB-A-2 261 247 discloses a solar collector comprising amultiple glazing unit beneath which an absorber is placed. The multipleglazing unit comprises at least one pair of glass sheets spaced apart bya metal spacer welded to the glass sheets via strips of conductiveenamel.

One drawback of this solar collector is that the absorber and the pipesin which the heat-transfer fluid circulates are placed beneath themultiple glazing unit. The solar collector therefore has a largethickness, which causes handling problems when fitting it. Moreover, itis necessary to add a thermally insulating coating beneath the absorberand the pipes so as to minimize the heat losses. This complicates thesolar collector and further increases the thickness of the solarcollector. In addition, the solar radiation must pass through at leasttwo glass sheets in order to reach the absorber, thereby reducing thetransmission of the solar radiation.

There is therefore a need for a solar collector which is compact andsimple and improves the transmission of solar radiation to the absorber.

To do this, the invention provides a solar collector, comprising:

a glass sheet provided with a fired metal frit;

a metal frame or another glass sheet provided with a fired metal fritand a metal frame;

a brazed seal between the metal frit or frits and the metal frame;

an absorber and pipes in which a heat-transfer fluid circulates, thepipes being in contact with the absorber, and the absorber and the pipesbeing placed between the glass sheet and the metal frame or between thetwo glass sheets.

According to another feature, the solar collector comprises a singleglass sheet and a metal frame, the metal frame being provided with abottom and the free edge of the metal frame being brazed to the metalfrit of the glass sheet.

According to another feature, the solar collector comprises two glasssheets and a metal frame, the edges of the metal frame being brazed tothe metal frits of each of the two glass sheets.

According to another feature, the metal frame comprises a wall offsetwith respect to the brazed seal and/or a wall connected to the brazedseal by a low thermal conductivity material.

According to another feature, the glass sheet(s) is (are) tempered.

According to another feature, the brazing alloy has a melting pointbetween 100° C. and 350° C.

According to another feature, the brazing alloy is the alloyPb_(93.5)Sn₅Ag_(1.5).

According to another feature, the fired metal frit comprises between 50%and 95% by weight of silver particles, the rest being a vitreous bindercomprising SiO₂, Bi₂O₃, Na₂O and ZnO.

According to another feature, the solar collector is under a vacuum.

According to another feature, the glass sheet is made of extra-clearglass.

According to another feature, the glass sheet is provided with anantireflection coating.

According to another feature, each glass sheet is joined to anadditional glass sheet via a polymeric interlayer in order to form alaminated glazing unit.

Another object of the invention is to provide a solar collectorcomprising:

a glass sheet;

a metal frame or another glass sheet and a metal frame;

a seal between the glass sheet or sheets and the metal frame,

the metal frame comprising a wall offset with respect to the brazed sealand/or a wall connected to the brazed seal by a low thermal conductivitymaterial.

Other features and advantages of the invention will now be described inconjunction with the drawings in which:

FIG. 1 shows a cross-sectional view of a solar collector according to afirst embodiment of the invention;

FIG. 2 shows a cross-sectional view of a solar collector according to asecond embodiment of the invention; and

FIG. 3 shows a cross-sectional view of a solar collector according to athird embodiment of the invention.

The reference numbers that are the same on the various figures representidentical or similar elements.

The invention relates to a solar collector comprising a glass sheetprovided with a fired metal frit. The solar collector also comprisesanother glass sheet provided with a fired metal frit and/or a metalframe.

A brazed seal is produced between the metal frit or frits and the metalframe or between the metal frits of the two glass sheets. Thus, thesolar collector is hermetically sealed, thereby making it possible inparticular to maintain a vacuum in the solar collector.

The solar collector also comprises an absorber and pipes in which aheat-transfer fluid circulates. The pipes are in contact with theabsorber so as to maximize the heat exchange between the absorber andthe heat-transfer fluid. In addition, the absorber and the pipes areplaced between the glass sheet and the metal frame, or between the twoglass sheets.

Thus, the solar collector is compact since the absorber and the pipesare integrated between the glass sheet and the metal frame, or betweenthe two glass sheets. In addition, the solar collector is simple, as itavoids the use of an additional insulating coating. The solar radiationhas only a single glass sheet to pass through in order to reach theabsorber. This makes it possible to improve the transmission of thesolar radiation to the absorber.

FIG. 1 shows a cross-sectional view of a solar collector according to afirst embodiment of the invention.

In this embodiment, the solar collector comprises a glass sheet 1 and ametal frame 2 sealed onto the glass sheet 1. The seal is produced bybrazing with the aid of a brazing alloy 4 via a metal frit 3 depositedon the glass sheet 1. Such a seal between the glass and the metal ismechanically strong and remains impermeable. This seal is particularlyadvantageous when the solar collector is under a vacuum, since itprevents the vacuum from deteriorating over the course of time.

The metal frit 3 is deposited on the perimeter of one face of the glasssheet 1, preferably by screen printing. Deposition by screen printing isin fact simpler than thin-film deposition in the context ofindustrialization.

The metal frit 3 is dried at 80° C. The glass sheet 1 provided with themetal frit 3 are then heated to a temperature between 400° C. and 700°C. so as to fire the metal frit 3. This firing temperature makes itpossible for the glass sheet 1 not to be damaged. The glass sheet 1provided with the fired metal frit is then cooled to room temperature.

If the glass sheet 1 of the solar collector is made of tempered glass,the firing of the metal frit 3 is carried out during the thermaltempering of the glass sheet. The firing temperature of the frit is thenpreferably above 600° C. and the cooling takes place with the aid of aplurality of nozzles injecting compressed air close to said glass sheet.The final surface stress in the glass is then for example 120 MPa, for aglass 4 mm in thickness, and the silver frit is fired.

The solar collector may be intended to be installed on a roof, forexample to heat water for domestic applications. The fact that the glasssheet 1 is made of tempered glass enables the mechanical properties ofthe glass to be increased, so that the glass sheet 1 is more resistantto foul weather, for example resistant to hailstones, and to themechanical stresses induced by atmospheric pressure on the solarcollector and by thermal expansion of the glass sheet of the solarcollector.

Note that the step of depositing the metal frit by screen printing onthe perimeter of one face of the glass sheet 1 is particularly wellintegrated into an industrial thermal tempering line.

The fired metal frit 3 comprises between 50% and 95% by weight of silverparticles, the rest being a vitreous binder. Thus the fired metal fritconsists for example of 94% by weight of silver particles and 6% byweight of vitreous binder comprising SiO₂, Bi₂O₃, Na₂O and ZnO.

The metal frit 3 adheres perfectly well to the glass sheet 1 and is thusparticularly well suited to brazing with another metallic element,namely here the metal frame, so as to form a hermetic seal.

The metal frame 2 comprises a bottom 20 and a free edge 21 intended tobe brazed to the metal frit 3. Preferably, the free edge 21 and themetal frit 3 are descaled before brazing, thereby enabling a better sealto be obtained.

The bottom 20 of the metal frame is also metallic. This makesmanufacture easier since the edge 21 and the bottom 20 may be made asone piece, for example by deep drawing, or else they may beconventionally welded.

The solar collector also comprises an absorber 5 and pipes 6 in contactwith the absorber. The absorber 5 is designed to absorb the solarradiation transmitted through the glass sheet 1. For example, theabsorber 5 is a metal plate covered with a low-emissivity coating. Themetal allows good solar radiation absorption, while the low-emissivitycoating ensures that the least possible amount of solar radiation isreemitted to the outside of the solar collector.

A heat-transfer fluid 7, for example water, circulates in the pipes 6.The heat coming from the solar radiation that passes through the glasssheet 1 is transmitted from the absorber 5 to the pipes 6 and then tothe heat-transfer fluid 7.

The absorber 5 and the pipes 6 are placed inside the metal frame 2. Thefired metal frit 3 of the glass sheet 1 is then brazed onto the freeedge 21 of the metal frame 2 with the aid of a brazing alloy so as toseal the solar collector. The absorber 5 and the pipes 6 are kept at acertain distance from the glass sheets, for example using spacers (notshown). These spacers also make it possible to withstand the pressuredifference between the external air and the internal vacuum.

The brazing alloy used preferably has a melting point between 100° C.and 350° C. If the glass sheet 1 is made of tempered glass, thisrelatively low melting point prevents the glass from becomingdetempered. Moreover, it prevents the low-emissivity properties of theabsorber 5 from deteriorating and limits the mechanical stresses inducedby the difference in thermal expansion coefficient between the glass andthe metal.

If the solar collector is to be under a vacuum, this is created between100 and 300° C., after the brazing step. This is because creating avacuum is more effective if it is done at high temperature, therebyaccelerating the desorption and increasing the pressure inside the solarcollector. Creating the vacuum within the solar collector providesexcellent insulation between the absorber 5 and the external medium bycutting out convective and conductive heat transfer in the internal air.This greatly improves the efficiency of the solar collector obtained.

Preferably, a melting point of the brazing alloy between 250 and 350° C.is a good compromise between the need to heat when creating the vacuumwithout remelting the brazing alloy and the need not to heat the glasstoo much so as not to detemper it.

The brazing alloy is for example the alloy Pb_(93.5)Sn₅Ag_(1.5), whichhas a melting point of 300° C.

The glass sheet 1 may also comprise a low-emissivity coating on itssurface, preferably on the inside of the solar collector so that it isnot degraded due to foul weather. This low-emissivity coating may bedeposited on the glass sheet before or after the metal frit has beendeposited.

The glass of the glass sheet 1 may be an extra-clear glass so as tominimize the absorption of solar radiation and thus maximize its energytransmission. The glass sheet 1 may also be provided with anantireflection coating on its external face.

In an installation on a building roof, the glass sheet 1 is turned tothe outside of the building, whereas the bottom 20 of the metal frame 2is turned toward the building.

This first embodiment has the advantage over the second and thirdembodiments, which will be described below, of having only a singlebrazed seal, thereby limiting the risk of leakage.

FIG. 2 shows a cross-sectional view of a solar collector according to asecond embodiment of the invention.

In this embodiment, the solar collector comprises two glass sheets 1 anda bottomless metal frame 8. The bottomless metal frame 8 serves as aspacer between the two glass sheets 1. The edges 81 of the metal frame 8are each brazed to the fired metal frit 3 of one of the glass sheets 1so as to seal the solar collector.

The two glass sheets 1 may be made of tempered glass. As a variant, onlyone of the two glass sheets—the one through which the sun's rays passdirectly—is made of tempered glass.

In an installation on a building roof, one or other of the two glasssheets 1 may be turned toward the outside of the building, the otherglass sheet being turned toward the building.

Moreover, all that was stated in respect of the first embodiment remainsvalid in respect of this second embodiment.

This second embodiment has the advantage of making the structuresymmetrical, thereby preventing the collector from bending when it issubjected to temperature variations caused by the difference in thermalexpansion coefficient between the glass and the metal, for exampleeither when creating the vacuum or during use.

FIG. 3 shows a cross-sectional view of a solar collector according to athird embodiment of the invention.

This embodiment is a variant of the second embodiment. Only the metalframe 8 differs from the second embodiment. The metal frame 8 consistsof a wall 80 approximately perpendicular to the glass sheets 1 and ofturned-over edges 81 approximately parallel to the glass sheets 1. Theedges 81 and the wall 80 may be made of one piece, for example by deepdrawing, or else the edges 81 may be attached to the wall 80.

The third embodiment includes a metal frame especially designed tominimize heating of the brazed seal during use of the solar collector.

In fact the pipes 6 of the solar collector pass through the wall 80 ofthe metal frame in a sealed manner so as to enter and leave the solarcollector. The pipes are at a high temperature, for example around 80°C. in the case of domestic applications and around 170° C. in the caseof refrigeration applications with two-stage absorption machines. Thewall 80 will therefore be heated by the pipes 6. It is advantageous tolimit heat exchange between the wall 80 and the brazed seal 4 so as notto damage the latter, so that it can remain vacuum-tight as long aspossible, so as to guarantee the longevity of the solar collector.

A first solution for limiting heat exchange between the wall 80 and thebrazed seal 4 is for the edges 81, and optionally the wall 80, of themetal frame 8 to have a low thermal conductivity, for example by beingmade of a low thermal conductivity material and/or by having a smallthickness. Thus, the edges 81 and optionally the wall 80 preferably havea thermal conductivity of less than 20 W/m/K, more preferably less than15 W/m/K and ideally less than 1 W/m/K. For example, they are thereforemade of stainless steel or else the alloy referred to by the trade markKovar®, which has a thermal conductivity of 17 W/m/K. Likewise, thethickness of the edges 81 and optionally of the wall 80 is preferablyless than 1 mm, more preferably less than 0.5 mm.

A second solution for limiting heat exchange between the wall 80 and thebrazed seal 4 is to offset the wall 80 by a distance of at least 1 cm,preferably 2 cm, from the brazed seal 4 with the aid of the edges 81. Inthis instance, the wall is offset over its entire height.

The first and second solutions may be combined on the same metal frameso as to further reduce heat exchange between the wall 80 and the brazedseal 4.

Thus, the second embodiment provides a compact solar collector whereasthe third embodiment enables the performance of the solar collector tobe optimized.

The action of offsetting the wall of the metal frame with respect to theglass/metal seal and/or of reducing the thermal conductivity of theedges of the metal frame, by choosing a suitable material and/or byreducing the thickness of the edges and optionally of the wall of themetal frame, may be applied to a solar collector in which the glasssheet or sheets are sealed to the metal frame by another conventionaltechnique, namely a technique other than by brazing via a metal frit.

The action of offsetting the wall of the metal frame with respect to theglass/metal seal and/or of reducing the thermal conductivity of theedges of the metal frame, by choosing a suitable material and/or byreducing the thickness of the edges and optionally of the wall of themetal frame, may be applied to the embodiment shown in FIG. 1.

The spacers (not shown), which it possible to withstand the pressuredifference between the external air and the internal vacuum, compensatefor the loss of compressive strength of the wall 8 due to its smallthickness and/or the fact that it is offset, and thus prevent it frombowing under the effect of the vacuum, so as to maintain the structureof the collector.

In the three embodiments, each glass sheet 1 may be combined with anadditional glass sheet via a polymeric interlayer so as to form alaminated glazing unit. This results in greater personal safety vis àvis the risk of an implosion inherent in any glass system under vacuum.

Thus, the solar collector comprises one or two glass sheets 1 and ametal frame 2 or 8, and a seal between the glass sheet or sheets 1 andthe metal frame 2, 8. The metal frame 8 comprises a wall 80 offset withrespect to the seal and/or a wall 80 connected to the seal by a lowthermal conductivity material. The wall is offset over its entireheight.

The solar collector further comprises an absorber 5 and pipes 6 in whicha heat-transfer fluid 7 circulates, the pipes 6 being in contact withthe absorber 5. The absorber 5 and the pipes 6 are placed between theglass sheet 1 and the metal frame 2 or between the two glass sheets 1.

In the embodiment in which the solar collector comprises a single glasssheet 1 and a metal frame 2, the metal frame 2 is provided with a bottom20 and the free edge 21 of the metal frame 2 is sealed to the glasssheet 1.

In the embodiment in which the solar collector comprises two glasssheets 1 and a metal frame 8, the edges 81 of the metal frame 8 aresealed to each of the two glass sheets 1.

The glass sheet(s) 1 may be tempered.

The solar collector may be under a vacuum.

The glass sheet(s) 1 may be made of extra-clear glass.

The glass sheet(s) 1 may be provided with an antireflection coating.

Each glass sheet 1 may be joined to an additional glass sheet via apolymeric interlayer in order to form a laminated glazing unit.

1. A solar collector, comprising: a glass sheet; a metal frame oranother glass sheet and a metal frame; a seal between the glass sheet orsheets and the metal frame; the metal frame comprising a wall offsetwith respect to the seal and/or a wall connected to the seal by a lowthermal conductivity material.
 2. The solar collector as claimed inclaim 1, further comprising an absorber and pipes in which aheat-transfer fluid circulates, the pipes being in contact with theabsorber, and the absorber and the pipes being placed between the glasssheet and the metal frame or between the two glass sheets.
 3. The solarcollector as claimed in claim 1, comprising a single glass sheet and ametal frame, the metal frame being provided with a bottom and a freeedge of the metal frame being sealed to the glass sheet.
 4. The solarcollector as claimed in claim 1, comprising two glass sheets and a metalframe, the edges of the metal frame being sealed to each of the twoglass sheets.
 5. The solar collector as claimed in claim 1, wherein theglass sheet is, or the glass sheet and the other glass sheet are,tempered.
 6. The solar collector as claimed in claim 1, the solarcollector being under a vacuum.
 7. The solar collector as claimed inclaim 1, wherein the glass sheet is made of extra-clear glass.
 8. Thesolar collector as claimed in claim 1, wherein the glass sheet isprovided with an antireflection coating.
 9. The solar collector asclaimed in claim 1, wherein each glass sheet is joined to an additionalglass sheet via a polymeric interlayer in order to form a laminatedglazing unit.